1. Field of the Invention
The present invention relates to an LCD (liquid crystal display) and a method of driving the same, and particularly, to an LCD employing a phase transition liquid crystal and a method of driving the LCD.
2. Description of the Related Art
LCD display units are frequently used in office automation equipment such as personal computers and word processors. As office automation advances, the LCDs are required to have a higher information content, a higher response speed, a wider viewing angle, a brighter screen, and an ability to display high-quality color images. Recently, projection LCDs have attracted attention because it is relatively easy for such LCDs to provide a large-sized image.
The projection LCDs presently marketed are TN (twisted nematic) LCDs employing TFTs (thin film transistors). The TFT LCDs are capable of applying a sufficient voltage to each pixels so that they theoretically achieve a high response speed and sufficient intensity levels required for displaying video images. The TFT LCDs are expected to serve as large full-color projection display units.
Conventional projection TFT-TN-LCDs, however, must employ a polarizing film, which darkens the projected image. The polarizing film absorbs light from a light source and changes it into heat, so that the projection TFT-TN-LCDs provide not only dark images but also low contrast due to the heat if the LCDs are insufficiently cooled. It is preferable, therefore, to drive the projection LCDs without using the polarizing film but using of the transmission and scattering of light.
The transmission-scattering LCDs employing no polarizing film are dynamic scattering LCDs, phase transition LCDs, and polymer dispersion LCDs (PDLCs).
The dynamic scattering LCDs employ a current effect to drive liquid crystal. Accordingly, they cannot be driven with amorphous silicon TFTs because of insufficient electron mobility.
The polymer dispersion LCDs use a driving voltage of several tens of volts so that they are not yet in practical use.
On the other hand, the phase transition LCDs are capable of providing bright display images without polarizing films. The driving voltage of the phase transition LCDs is 25 to 30V, which is about half of the driving voltage of the polymer dispersion LCDs. In addition, the phase transition LCDs employ an electric field effect, so that they may in principle be driven with amorphous silicon TFTs.
It is understood, therefore, that the LCDs employing phase transition liquid crystal overcome the drawbacks of the TFT-TN-LCDs and serve full-color projection LCDs that provide high-quality images. The phase transition LCDS, however, have the problems mentioned below so that quality images are not provided.
Namely, the conventional LCDs employing phase transition liquid crystal have the following problems to be solved:
(1) The phase transition LCDs involve a relatively high driving voltage of 25 to 30V, so that they cannot be driven by practical amorphous silicon TFTs whose driving voltage is about 10V.
(2) Changing the phases of the phase transition liquid crystal from one to another involves electro-optic hysteresis, which puts a limit on intensity levels in the displayed images.
(3) When the liquid crystal material is changed to another that can be driven at about 10V, substantially no contrast will be produced.
(4) When the liquid crystal material is changed to another that can be driven at about 10V, the long response time will not allow video images to be displayed.